This invention relates to a top seal for a rotary valve for services requiring a sealing member which is extremely stable and which provides an effective seal for a wide range of temperature, pressure and loading conditions. PTFE has been widely used as a material for this purpose. However, it has a tendency to cold flow under load. Valve components may be secured tightly in assembly to insure sealing, but at high temperatures the PTFE tends to soften and it tends to squeeze out from between such components, often leaving a void in the seal. In addition, because PTFE has a low degree of resiliency, it is difficult to provide an effective seal unless backed up by a pressure inducing member. In the case of sealing around the stem or other parts of a rotatable valve member, excessive clamping force will impede rotation of the valve member.